The present invention relates to cemented carbides and a method and device for drying powder mixtures of cemented carbide and the like.
In the discussion of the state of the art that follows, reference is made to certain structures and/or methods. However, the following references should not be construed as an admission that these structures and/or methods constitute prior art. Applicant expressly reserves the right to demonstrate that such structures and/or methods do not qualify as prior art against the present invention.
Cemented carbide is made by powder metallurgical methods consisting of the wet milling in an alcohol-water-solution of a powder mixture containing powder forming the hard constituents and a binder phase, drying of the milled mixture to a powder with good flow properties by spray drying, pressing of the dried powder to bodies of desired shape and finally sintering.
The intensive milling operation is performed in mills of different sizes using cemented carbide milling bodies. Milling is considered necessary to obtain a uniform distribution of the binder phase in the milled mixture. The milling time can vary from several hours up to days. The milling operation produces a slurry suitable for spray drying. Successful spray drying depends strongly on the properties of the slurry. The present technology with intensive milling under long periods of time usually gives a very fine-grained powder whose sedimentation rate is sufficiently slow. As result of the spray drying a ready-to-press powder is obtained consisting essentially of spherical agglomerates of about 0.1 mm mean size.
It is desirable that the spray-dried powder shall be in the form of spherical well-defined agglomerates with a limited variation of the sizes of the agglomerates. Thus, a more even powder density is obtained, giving improved flow properties as well as a reduction of filling variations in the subsequent pressing operation. The distribution or size variations can be influenced by optimising the alcohol to water ratio and liquid to powder ratio in the slurry, the amount of pressing agent etc. Another way is to modify the nozzles in the spray drier. A variant of such a nozzle is described in U.S. Pat. No. 4,978,069. This patent relates to the generation of even drops from a melt, but it can also be applied to drying of cemented carbide powder. However, it has been found that this does not give the desired limited variation of the sizes of the agglomerates. It has been found better to use a simpler nozzle instead, and expose the powder to a fractioned sieving after the drying process to eliminate the relatively coarse and fine part of the agglomerates to obtain the desired narrow distribution of sizes. The distribution after the sieving must be controlled since there is a risk of clogging of the net of the sieve, which may influence the results. The sieving also adversely affects the properties of the powder. However, the distribution of sizes of the agglomerates is normally so broad that the final distribution cannot be made as narrow as desirable for economical reasons. A way to describe distribution of the sizes of the agglomerates is through the expression d97/d03 where d97=the grain size below which 97% of the agglomerates are found and d03=the grain size below which 3% of the agglomerates are found. A d97/d03value=4 has been found to be acceptable. However, it is desirable to obtain an even lower value, preferably without a subsequent sieving operation.
It is an object of the invention to overcome the disadvantages of the prior art.
It is an object of the present invention to provide a method for the production of cemented carbide powder with a limited distribution of grain sizes d97/d03 less than 4, without subsequent sieving.
Surprisingly it has now been found that if the apparatus, according to U.S. Pat. No. 4,978,069 is modified in the way more closely described below a cemented carbide powder is obtained with the desired limited distribution of grain sizes.
In an apparatus for drop forming by disks, according to the present invention, jets of the liquid are ejected towards a first funnel-shaped part where the liquid is accelerated in a first step after which the jets are permitted to spread somewhat, they are then ejected over to a second funnel-shaped part where the liquid is accelerated in a second step to the speed of the drop forming disk. The acceleration of the liquid is thus divided into two steps. During the second acceleration step the acceleration is increased because the spreading of the jet results in increased surface contact.
At the periphery of the drop forming disk the risk that contact between the surface of the disk and the liquid be broken is minimized thanks to the fact that the outermost part of the disk is angled relative to the plane of rotation in such a way that the centrifugal force acting on the liquid is split into one component directed along the surface of the disk and one directed against the surface of the disk. In this way the contact between the surface of the disk and the layer of liquid right up to the means of drop forming is ensured.
According to one aspect, the present invention provides a method of making powders from a slurry, the method comprising the steps of: (i) introducing the slurry into a drop forming apparatus; (ii) ejecting the slurry from a discharge opening to form a jet such that the jet hits an inwardly and downwardly oriented first oblique rotating surface; (iii) rotating the first surface at a speed such that the jet is forced upwards over the first surface and over another connected horizontal surface by centrifugal forces; (iv) dividing and directing the jet towards an outwardly and downwardly oriented second oblique rotating surface; (v) rotating the second surface such that the slurry in the jet is accelerated to the speed of rotation of the second surface; (vi) directing the jet onto an outwardly oriented third oblique rotating surface, from which drops of the slurry detach themselves when centrifugal forces on the third surface exceeds adhesive forces between the drops and the third surface.
According to another aspect, the present invention provides an apparatus for forming drops from a slurry, the apparatus comprising: a plurality of rotatable disk units, the disk units comprising a number of disks arranged axially on top on each other, the disks have a cross-section comprising a radially inner section with a substantially U-shaped recess constructed to receive a raised L-shaped section of an adjacent disk with sufficient play therebetween to allow radial ejection of a jet of the slurry; and a distributor for uniformly distributing the slurry onto the plurality of disk units.
According to yet another aspect, the present invention provides powdered agglomerates of cemented carbide, cermets, ceramics or similar materials with abrasive wear resistance wherein the agglomerates have a distribution of size, prior to sieving, such that a ratio of d97/d03 less than 4 wherein
d97=the grain size below which 97% of the agglomerates is found and d03=the grain size below which 3% of the agglomerates is found.